Image forming apparatus with paper thickness detection unit and image forming method of the same

ABSTRACT

An image forming apparatus of an embodiment of the invention includes a paper feed cassette to contain a sheet on which an image is to be printed, a pair of conveyance rollers to nip and convey the sheet taken out from the paper feed cassette, a paper thickness detection unit that is provided downstream of the sheet conveyed by the pair of conveyance rollers and near a nip point between the pair of conveyance rollers, has a rotator brought into contact with the sheet, and detects a thickness of the sheet by a shift generated when the sheet contacts with the rotator, a register roller to align the sheet after the thickness of the sheet is detected by the paper thickness detection unit, and a print ejection mechanism that prints and fixes an image to the sheet passing through the register roller and then ejects it.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on the benefit of the priority of provisional application No. 60/969,144 filed on Aug. 30, 2007, the benefit of the priority of provisional application No. 60/971,239 filed on Sep. 10, 2007, and the benefit of the priority of provisional application No. 60/972,241 filed on Sep. 13, 2007. The contents of these provisional applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus, and particularly to an image forming apparatus with a paper thickness detection unit, and a method of the same.

BACKGROUND

Hitherto, in an image forming apparatus such as a multi-function color copier (MFP), the types of media on which color images or the like are printed are increased. Even when the print medium is limited to paper, various paper sheets different from each other in thickness or the like are used.

Incidentally, the paper types are generally classified by paper weight (unit: g/cm²) indicating the weight per area. For example, sheets of groups of 64 to 105, 106 to 163, 164 to 209, 210 to 256, and 257 to 300 are called standard paper, thick paper 1, thick paper 2, thick paper 3, thick paper 4, respectively. These paper weights are generally written on packages of sheets. The user selects one of the written groups of the paper weights, so that a print condition corresponding to the type of the paper is automatically set.

However, the paper weight can be calculated from the density and the paper thickness of a sheet, and the paper type can be generally detected by measuring the paper thickness. Thus, there is known an image forming apparatus with a paper thickness detection mechanism that can detect the paper thickness.

One of these paper thickness detection apparatuses includes a pair of conveyance rollers to convey a sheet, measures a displacement amount of the conveyance rollers when the sheet passes through between the conveyance rollers, and detects the paper thickness from the displacement amount. That is, the conveyance rollers are used also as the paper thickness detection rollers. However, in this kind of apparatus, since the conveyance. rollers have the function to convey the sheet, they are required to move the passing sheet and must apply a specified pressure to the sheet. Accordingly, the sheet whose paper thickness is detected is pressed, and it is difficult to detect the accurate paper thickness.

In another one of the paper thickness detection apparatuses, a roller dedicated to paper thickness detection is provided in addition to a conveyance roller, an opposite plate is provided to be opposite to the roller, and a displacement amount of the paper thickness detection roller, which is caused by a sheet entering between the opposite plate and the paper thickness detection roller, is measured to detect the paper thickness. However, in this kind of apparatus, since the paper thickness detection roller is not driven, when the sheet is made to enter the paper thickness detection roller, unless the pressing force of the paper thickness detection roller to the sheet is made small, the entering sheet is buckled. However, when the pressing force of the paper thickness detection roller is made small, holding by the paper thickness detection roller becomes unstable, and an external vibration is liable to exert an influence. Besides, when the pressing force of the paper thickness detection roller is made small, the paper thickness detection roller is likely to be bounded by an impact generated when the sheet enters.

In a still another one of the paper thickness detection apparatuses, for example, as disclosed in JP-A-2003-237982, in the above paper thickness detection apparatus, a drive roller is provided instead of the opposite plate, and this drive roller is rotated in synchronization with the conveyance of the sheet. However, in this kind of paper thickness detection apparatus, there arise problems that the detection of accurate paper thickness becomes difficult by the influence of vibration due to the rotation of the drive roller when the paper thickness is detected, and the cost is increased since the drive roller is required.

Incidentally, JP-A-7-187452 discloses a sheet conveying apparatus in which a paper thickness detection roller is provided downstream of a conveyance roller to convey a sheet contained in a paper feed cassette and upstream of a register roller to align the sheet.

SUMMARY

The present invention is made in view of the above, and provides an image forming apparatus with a paper thickness detection unit that has a simple structure, and can accurately detect paper thickness without increasing the cost, and an image forming method.

According to an aspect of the invention, an image forming apparatus includes a paper feed cassette to contain a sheet on which an image is to be printed, a pair of conveyance rollers to nip and convey the sheet taken out from the paper feed cassette, a paper thickness detection unit that is provided downstream of the sheet conveyed by the pair of conveyance rollers and near a nip point between the pair of conveyance rollers, has a rotator brought into contact with the sheet, and detects a thickness of the sheet by a shift generated when the sheet contacts with the rotator, a register roller to align the sheet after the thickness of the sheet is detected by the paper thickness detection unit, and a print ejection mechanism that prints and fixes an image to the sheet passing through the register roller and then ejects it.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the outline of a multi-function color copier (MFP) of an embodiment of the invention.

FIG. 2 is a view showing an example of a display screen of a touch panel display 103 of an operation panel 102 when paper weight is inputted.

FIG. 3 is a view showing the whole electrical rough structure of the embodiment.

FIG. 4 is a view showing a rough structure of the MFP of the embodiment of the invention in which the flow (supply, thickness detection, print, post-processing) of a sheet is mainly illustrated.

FIG. 5 is a view for explaining a relation between a conveyance drive mechanism and a paper thickness detection mechanism in the embodiment.

FIG. 6 is a view for explaining that a conveyance guide 301B and the like can be separated from a conveyance guide 301A and the like in a lateral direction.

FIG. 7 is a view for explaining that a conveyance driven roller and the like can be separated from the conveyance guide 301B in the embodiment.

FIG. 8 is a view showing a structure of a paper thickness detection unit 124 in the embodiment.

FIG. 9 is a perspective view showing a structure of a resistance change detection module 401, an arm 403 and a bearing 404 in the paper thickness detection unit 124.

FIG. 10 is a view showing an example of an output voltage of a voltage detection circuit 406 when a sheet enters.

FIG. 11 is a view showing a relation between a sheet feed mechanism and a conveyance drive roller 206A and a conveyance guide 301A.

FIG. 12 is a view showing a relation between an operation timing of a paper feed drive motor and a paper thickness detection timing in another embodiment for preventing vibration at paper thickness detection.

FIG. 13 is a view showing a relation between the conveyance guide 301A and frames 504 a and 504 b in a still another embodiment for preventing vibration at paper thickness detection.

FIG. 14 is a view showing a connection relation between the conveyance guide 301A and a stay 50 in a still another embodiment for preventing vibration at paper thickness detection.

FIG. 15 is a view showing a relation between a holding mechanism 309 and the conveyance guide 301B in a still another embodiment for preventing vibration at paper thickness detection.

DETAILED DESCRIPTION

Hereinafter, an embodiment of an image forming apparatus of the invention will be described with reference to the drawings. In the following, a description will be made on the assumption that the image forming apparatus of the embodiment is a multi-function color copier.

FIG. 1 is a perspective view showing an example of the outer appearance of the multi-function copier of the embodiment of the invention. An auto document feeder (ADF) 101 that is used also as a document cover and automatically feeds sheet-like original documents one by one is openably and closably provided at an upper part of an apparatus body 100. An operation panel 102 including various operation keys to instruct a copy condition and a copy start, and various displays is provided at an upper front part of the apparatus body 100. A touch panel display 103 that displays various information to a user and can perform a specified input by user's touch when input is required is provided on the side of the operation panel 102.

A handle 104 is provided at the front of the apparatus body 100 and at a lower part of the operation panel 102 so that the inside of the body can be opened at paper jam or the like.

Paper feed cassettes 111, 112, 113 and 114 are detachably and attachably provided at a lower part of the apparatus body 100. Sheets for color printing and sheets for monochrome printing, which are equal to each other in size and are different from each other in paper type, are contained in the respective paper feed cassettes, and are selected and fed when printing is performed as described later.

A post-processing apparatus 115 is put in contact with the left side of the apparatus body 100. In the apparatus body 100, as described later, a latent image is formed and is printed and fixed to a sheet, and the sheet is subjected to processes such as alignment and stapling in the post-processing apparatus 115, and is ejected from a sheet ejection port 116. The sheet ejected from the sheet ejection port 116 is loaded on a loading tray 117.

FIG. 2 shows an example displayed on the touch panel display 103. First, “Please select a sheet type” is displayed at an upper part, and sheet types as display buttons are displayed below it.

Buttons P0, P1 and P2 of standard paper (auto), standard paper 1 and standard paper 2 are arranged and displayed at the first stage, buttons of four types of thick papers, that is, thick paper 1, thick paper 2, thick paper 3 and thick paper 4 are displayed at the second stage. Buttons corresponding to types of sheets other than the standard paper and the thick paper are displayed at the third stage. In the example shown in FIG. 1, since the four paper feed cassettes exist, for example, two types of standard papers (standard paper 1 and standard paper 2) and two types of thick papers (thick paper 1 and thick paper 2) are supplied to these paper feed cassettes. The corresponding display buttons P1, P2, TH1 and TH2 are displayed with black frames and can be selected. The operator touches one of the black frame display buttons, and then touches a setting display button S1, so that the sheet to be recorded is determined.

The display button P0 of the standard paper (auto) is also displayed with a black frame, and it is determined whether the standard paper is standard paper 1 or standard paper 2. The determined standard paper is supplied from the corresponding paper feed cassette.

FIG. 3 shows an electrical rough structure of the embodiment shown in FIG. 1. An MFP 119 includes a communication unit 120 connected to the outside through a network, a control panel control unit 121 to control the whole control panel including the operation panel 102 and the touch panel display 103 shown in FIG. 1, a cassette sheet correspondence recognition unit 122 to previously recognize types of sheets contained in the paper feed cassettes 111, 112, 113 and 114, a sheet supply control unit 123 to supply a sheet of a type corresponding to the sheet type inputted to the touch panel display 103 as explained in FIG. 2, a paper thickness detection unit 124 to accurately detect the thickness of the sheet supplied by the sheet supply control unit 123, a latent image generation unit 125 to scan an original document when copying or the like is performed in the MFP and to generate, for example, an electrostatic latent image, a development transfer unit 126 to develop the latent image formed in the latent image generation unit 125 with, for example, toner and to transfer the toner image to a specified sheet, a fixing unit 127 to fix the transferred image by a specified voltage, and a main control unit 128 to control the respective units.

The fixing unit 127 includes a fixing process unit 127 a to perform a fixing process of an image to a specified sheet, and a fixing temperature control unit 127 b to control a fixing temperature when the fixing process is performed.

FIG. 4 shows a rough structure of the MFP of the embodiment in which the flow of a sheet is mainly illustrated.

The paper feed cassettes 111, 112, 113 and 114 contain standard paper 1, standard paper 2, thick paper 1 and thick paper 2. The sheets contained in these paper feed cassettes are taken out one by one by paper feed rollers 201, 202, 203 and 204 as the need arises, and are supplied to a paper thickness detection conveyance unit 205. The sheet supply control unit 123 shown in FIG. 3 also includes a circuit to drive the paper feed rollers 201, 202, 203 and 204.

The paper thickness detection conveyance unit 205 includes the paper thickness detection unit 124 to detect the thickness of a conveyed sheet 118, and two pairs of conveyance rollers to convey the sheet, that is, two conveyance drive rollers 206A and two conveyance driven rollers 206B as described later. The sheet whose thickness was detected by the paper thickness detection unit 124 of the paper thickness detection conveyance unit 205 is conveyed, and is aligned by a pair of register rollers 207 a and 207 b. The sheet aligned by the register rollers 207 a and 207 b is supplied to the development transfer unit 126. The electrostatic latent image generated in the latent image generation unit 125 shown in FIG. 2 is developed with toner in the development transfer unit 126, and is transferred to the conveyed sheet.

The sheet to which the toner image was transferred is subjected to an image fixing process in the fixing unit 127, that is, subjected to printing. The printed sheet is ejected from the apparatus body 100 through some pairs of conveyance rollers 208, and enters the post-processing apparatus 115. The sheet entering the post-processing apparatus 115 is subjected to various post-processes (not shown), such as stapling, in the post-processing apparatus 115, is ejected from the sheet ejection port 116, and is loaded on the loading tray 117.

Next, the paper thickness detection conveyance unit 205 in this embodiment of the invention will be described. FIG. 5 shows a sectional structure of the paper thickness detection conveyance unit 205. The conveyance drive roller 206A is the roller, at least the peripheral surface of which is formed of, for example, rubber, and is rotated and driven by a conveyance drive motor 300. The conveyance driven roller 206B has a peripheral surface formed of, for example, plastic and is rotated in accordance with the rotation of the conveyance drive roller 206A.

The sheet passes between a conveyance guide 301A and a conveyance guide 301B. Roughly, the conveyance guide 301A has a section of a “substantially inverse C” shape, and the conveyance guide 301B has a section of an L-shape. The sheet 118 is conveyed upward by the conveyance drive roller 206A and the conveyance driven roller 206B. In order to enable the sheet 118 to be easily removed when it is jammed on the way, the conveyance guide 301B can be moved in a lateral direction, that is, an arrow 305 direction.

FIG. 6 is a sectional view of a state where the conveyance guide 301B and the conveyance driven rollers 206B and 206B are separated from the conveyance guide 301A and the conveyance drive roller 206A. Further, FIG. 7 is a perspective view of a state where the conveyance driven rollers 206B and 206B are separated from the conveyance guide 301B in the lateral direction.

The conveyance guide 301A is provided with an opening 302 a, and the conveyance guide 301B is provided with an opening 302 b. The conveyance drive roller 206A and the conveyance driven roller 206B are in contact with each other through the opening 302 a and the opening 302 b. When the sheet 118 is supplied from the paper feed cassettes 111 to 114, the sheet is nipped between the conveyance drive roller 206A and the conveyance driven rolier 106B and is conveyed in an arrow direction (upward). As described later, the paper thickness detection unit 124 detects the thickness of the sheet in the middle of this conveyance.

An opening 306 provided between the two openings 302 b and 302 b of the conveyance guide 301B shown in FIG. 7 is the opening through which a bearing 404 (not shown in FIG. 7) of the paper thickness detection unit 124 described later comes in contact with the sheet 118.

As shown in FIG. 6, the conveyance guide 301B and the conveyance driven roller 206B can be separated from the conveyance guide 301A and the conveyance drive roller 206A. For example, when the sheet 118 is jammed in the vicinity of a place between the conveyance drive roller 206A and the conveyance driven roller 206B, the conveyance guides 301A and 301B are separated from each other as stated above, and the sheet can be removed.

Besides, as shown in FIG. 7, the conveyance guide 301B is attached to the body by, for example, pressing springs 308 a and 308 b, and is pressed in an arrow C direction. On the other hand, the conveyance driven roller 206B and a holding mechanism 309 thereof are provided independently of the conveyance guide 301B and the like. The reason is to prevent as far as possible that the vibration of the body described later is transmitted to the bearing 404 of the paper thickness detection unit 124 and, consequently, exerts an influence on the paper thickness detection.

FIG. 8 shows the whole structure of the paper thickness detection unit 124. The paper thickness detection unit 124 includes a resistance change detection module 401, an arm 403 that rotates around a fulcrum 402 of the resistance change detection module 401, the bearing 404 provided at the leading end of the arm 403, and the like. FIG. 9 is a perspective view showing a structure of the resistance change detection module 401, the arm 403 and the bearing 404.

The bearing 404 is pressed at a specified pressure by a not-shown spring or the like toward the direction of the conveyance guide 301A and the conveyance drive roller 206A. A pressing load P is, for example, 100 g. As shown in FIG. 5, a contact between the sheet 118 and the bearing 404 is provided downstream of a nip point between the conveyance drive roller 206A and the conveyance driven roller 206B. A distance D from the nip point between the conveyance drive roller 206A and the conveyance driven roller 206B to the nip position between the sheet 118 and the bearing 404 is, for example, about 6 mm.

If the pressing load P is made excessively large, when the sheet enters between the conveyance guide 301A and the conveyance guide 301B, the sheet 118 is not smoothly conveyed and is buckled. Besides, if the pressing load P is excessively small, the bearing 404 is not suitably brought into contact with the sheet, the bearing 404 is likely to be separated from the sheet by the vibration of a drive system, and the bearing 404 is separated from the sheet by an impact caused when the sheet 118 enters. Accordingly, if the pressing load P is excessively low, it becomes difficult to measure the accurate thickness of the sheet.

Besides, if the distance D is excessively large, the position where the bearing 404 contacts with the sheet becomes remote from the position where the sheet 118 is conveyed and driven, that is, the contact position between the conveyance drive roller 206A and the conveyance driven roller 206B. Since the bearing 404 does not have the conveyance function of the sheet 118, the conveyance force of the sheet becomes low at the position where the bearing 404 contacts with the sheet 118 even in such a state, and the normal conveyance of the sheet is likely to become difficult. As stated above, in general, there is a relation among the conveyance force of the sheet by the conveyance drive roller 206A and the conveyance driven roller 206B, the pressing load P of the bearing 404, and the distance D.

Accordingly, although the pressing load P of the bearing 404 varies by the conveyance force of the sheet and the material and structure of the bearing, the pressing load is preferably about 60 g to 140 g, and is more preferably about 80 g to 120 g. Besides, although the distance D varies by the conveyance force and the length of the contact portion between the conveyance drive roller 206A and the conveyance driven roller 206B, in general, it is appropriate that the distance is about 0 mm to 10 mm. Further, the distance D is preferably in the range of about 2 mm to 8 mm.

When the sheet 118 is conveyed along the conveyance guide 301A, the bearing 404 is rotated in a direction indicated by an arrow 405 a, and the arm 403 is shifted by the thickness of the sheet 118, that is, is slightly turned in a direction of an arrow 405 b. A magnet is provided near the fulcrum of the arm 403. A magnetic resistance sensor using magnetic resistance whose resistance value is changed correspondingly to the change of a magnetic field is provided near the magnet.

An electric signal output of the magnetic resistance sensor enters a voltage detection circuit 406, and its output voltage is sampled, for example, ten times in a sampling circuit 407. The reason why sampling is performed to average those values is that the bearing 404 is moved in the arrow 405 b direction by the vibration of the apparatus and the conveyance of the sheet, so that the value of the magnetic resistance is changed, and the output voltage of the voltage detection circuit 406 is changed.

The voltage values sampled in the sampling circuit 407 are averaged by the averaging circuit 408, and are inputted to a voltage difference detection circuit 409. The voltage different detection circuit 409 detects a difference between the averaged voltage values. The voltage difference corresponds to the thickness of the sheet 118. The magnetic resistance of the magnetic resistance sensor functions in the direction in which the resistance value is decreased when the sheet is conveyed, and the output voltage value of the voltage detection circuit 406 is decreased.

Setting is performed such that the voltage value detected by the voltage detection circuit 406 is 1 mV when the thickness of the sheet is 1 μm. In general, since the thickness of standard paper is about 100 μm, the standard paper is detected to be about 100 mV. For example, voltage V0 before the sheet passes is made 3.3 V, a value of about 1.35 V is made the center, and when the thickness of the sheet is thick, the voltage value is changed in the decreasing direction.

For example, it is assumed that the sheet 118 is nipped by the bearing 404 in a period from time T1 to T2, and the sheet is carried out at time T3 to return into the original state. As shown in FIG. 10, when there is no sheet 118, the voltage detection circuit 406 outputs the voltage of about V0. Also in this state, the output value fluctuates by the vibration of the apparatus or the like. The fluctuating output voltage value is sampled by the sampling circuit 407, and the sampled values are averaged by the averaging circuit 408. The averaged voltage value is sent to the voltage difference detection circuit 409, and V0 is once stored which is the voltage value when the sheet is not conveyed.

The sheet 118 is conveyed at time T1 and is nipped between the bearing 404 and the conveyance guide 301A. The bearing 404 is rotated as indicated by the arrow 405A, and is turned as indicated by the arrow 405 b. At this time, the value of the magnetic resistance in the magnetic resistance sensor is decreased, and the output value of the voltage detection circuit 406 becomes lower than V0 after time T2 as shown in FIG. 10.

Even in the state where the sheet 118 is nipped between the bearing 404 and the conveyance guide 301A and is moved, the bearing 404 is moved by the movement of the sheet 118 and the vibration of the apparatus. The output voltage value of the voltage detection circuit 406 is fluctuated by the movement of the bearing 404. These voltage values are sampled, for example, ten times in the sampling circuit 407 and are averaged by the averaging circuit 408. The averaged value is inputted to the voltage difference detection circuit 409, which is a voltage value V1 in the state where the sheet 118 is inserted.

The voltage detection circuit 409 outputs, as the voltage difference, the value obtained by subtracting the voltage value V1 from the previously detected voltage value V0. This value (V0−V1) corresponds to the thickness of the sheet 118, and the thickness of the sheet 118 is detected.

As stated above, when the paper thickness is detected as the difference between the voltage values, not as the voltage value, the offset of the voltage can be cancelled. Besides, such a problem that the voltage value is changed by distortion of the conveyance guide is removed. Accordingly, there is a merit that the paper thickness can be further accurately measured.

Since the vibration given to the bearing 404 of the paper thickness detection unit 124 exerts a bad influence on the detected voltage value, it is preferable that the vibration is as small as possible. One of main causes of the vibration is the vibration of the conveyance drive motor 300 to rotate and drive the conveyance drive roller 206A. Another one of the main causes of the vibration is the vibration of a sheet pickup conveyance motor (not shown) when the sheet is picked up from the paper feed cassette and is conveyed.

In this point, in the foregoing embodiment of the invention, the conveyance driven roller 206B, the holding mechanism 309 thereof and the conveyance guide 301B are provided independently of the body, and these mechanisms are provided to be separable from the conveyance drive roller 206A and the conveyance guide 301A. Accordingly, according to this embodiment, there is a merit that the influence of the vibration of the body exerted on the bearing 404 can be suppressed to be small.

As described above, in order to accurately detect the paper thickness, it is desired that the vibration given by the body to the bearing 404 of the paper thickness detection unit 124 is made as small as possible.

Other embodiments of the invention for decreasing the influence of the vibration exerted on the bearing will be described below. In order to decrease the vibration given to the bearing 404, these embodiments can also be combined.

Another embodiment of the invention relates to timings when the sheet 118 is conveyed from the paper feed cassettes 111, 112, 113 and 114 and when the paper thickness is detected.

FIG. 11 shows a sheet paper feed mechanism, a structure of a conveyance drive roller 206A and a conveyance guide 301A, and a paper thickness detection mechanism. A paper feed drive motor 501 is a motor for picking up and conveying a sheet from a paper feed cassette 201 by a sheet pickup roller 201P and paper feed rollers 201 a and 201 b.

The sheet is picked up from the paper feed cassette 111 by, for example, the paper feed roller 201 and the sheet pickup roller 201P, and is conveyed to a position between the conveyance drive roller 206A and a conveyance driven roller 206B through a paper feed conveyance roller 208 d, and then, the paper thickness is detected in a paper thickness detection unit 124.

FIG. 12 shows a relation between the timing of paper feed conveyance of the sheet and the timing of paper thickness detection. FIG. 12( a) shows a conventional paper feed conveyance timing, and shows that the paper feed drive motor 501 is driven in a state of H. FIG. 12( b) shows the timing of paper thickness detection, and the paper thickness is detected in a state of L, that is, in a period Td. As shown in FIG. 12( a) and FIG. 12( b), the timing of paper conveyance is irrelevant to the timing of paper thickness detection.

In this embodiment of the invention, the paper feed conveyance of the sheet is performed at the timing shown in FIG. 12( c). Also in this case, the state of H indicates that the paper feed drive motor 501 is driven. As is apparent from FIG. 12( b) and FIG. 12( c), in this embodiment, the paper feed drive motor 501 is not driven in the paper thickness detection period Td. The paper feed drive motor 501 is stopped in the paper thickness detection period, and the vibration thereof is not transmitted to the bearing 404.

As described above, when the paper feed of the sheet and the conveyance thereafter are not performed at the timing of paper thickness detection, the paper feed drive motor can be stopped during the paper thickness detection. As a result, the influence of the vibration to the bearing 404 can be reduced.

Next, a still another embodiment for reducing the influence of vibration of a motor and the like will be described with reference to FIG. 11 and FIG. 13. FIG. 13 is a plan view schematically showing a conveyance guide 301A and its peripheral structure. In general, as shown in FIG. 14, the drive guide 301A is fixed to a stay 503 by a screw 502, and the stay 503 is fixed to frames 504 a and 504 b of an apparatus body. In this embodiment of the invention, the conveyance guide 301A is not fixed to the stay 503, but is directly fixed to the frames 504 a and 504 b by support members 505 a and 505 b.

The vibration of the conveyance drive motor 300 is generally transmitted to the stay 503, is transmitted to the conveyance guide 301A, and is transmitted from the conveyance guide 301A to the bearing 404 of the paper thickness detection unit. As in this embodiment, when the conveyance guide 301A is directly connected to the frames 504 a and 504 b, the vibration from the stay 503 to the conveyance guide 301A can be suppressed.

Next, an embodiment for suppressing transmission of vibration to a bearing 404 will be described with reference to the drawings. FIG. 14 shows a structure in the embodiment. In general, a conveyance guide 301A is fixed to a stay 503 by a screw 502. Then, a vibration preventing material, for example, an elastic rubber (bush) 507 is inserted between the stay 503 and the conveyance guide 301A which are fixed by the screw 502.

The elastic rubber 507 suppresses the transmission of the vibration, which is transmitted to the stay 503, to the conveyance guide 301A. Accordingly, also by this, it is possible to prevent the vibration of the motor and the like from being transmitted to the conveyance guide, and the bearing 404 from the stay 503.

A still another embodiment for suppressing vibration transmitted to a bearing 404 is shown in FIG. 15. A structure of this embodiment suppresses the vibration which is transmitted from a holding mechanism 309 of a conveyance driven roller 206B to a conveyance guide 301B and is transmitted from the conveyance guide 301B to the bearing 404. In this embodiment, a vibration preventing material, for example, an elastic rubber 508 is fixed to a part where the holding mechanism 309 contacts with the conveyance guide 301B.

According to this embodiment, the vibration transmitted from the conveyance driven roller 206B and the holding mechanism 309 to the conveyance guide 301B can be suppressed.

When the thickness of the sheet is detected by the paper thickness detection unit 124 as stated above, the thickness of the sheet is reflected in the fixing temperature in the fixing unit 127 of FIG. 3.

The most frequently used standard paper is defined in the range of paper weight of 64 to 105 g/cm². However, in the case of a high-speed color copier, it is difficult to cover the wide range of paper weight by one kind of fixing temperature condition. Then, in this embodiment, according to the paper thickness, the paper with a thickness of up to 0.100 mm is identified as standard paper 1, and the paper with a thickness exceeding 0.100 mm and up to 0.130 mm is identified as standard paper 2.

When identified as standard paper 1, the temperature is controlled to become 150° C. to 175° C. by the fixing voltage control unit 127 b of the fixing unit 127 shown in FIG. 3. Besides, when identified as standard paper 2, the fixing temperature is controlled to become about 160° C. to 180° C. by the fixing voltage control unit 127 b.

When the type of the standard paper is not known, the user touches the icon P0 shown in FIG. 2. Then, as stated above, the paper thickness detection unit 124 operates to detect the paper thickness. In this way, when the user merely touches the icon of standard paper (auto), the fixing temperature is controlled to a suitable temperature, and the excellent image printing is obtained.

When the fixing temperature is not suitable for the sheet, there occurs a low temperature offset in which the temperature is low and the toner remains attached to the transfer roller, or a high temperature offset in which the fixing temperature is so high that the toner is peeled off. In such a case, the user touches the icon P1 or the icon P2, and selects standard paper 1 or standard paper 2.

As shown in FIG. 4, when the paper thickness detection unit 124 is provided on a common path after the sheet is conveyed from the plural paper feed cassettes 111, 112, 113 and 114, the paper thickness detection unit 124 can be used in common. Accordingly, it is unnecessary to provide the paper thickness detection unit for each of the paper feed cassettes.

Besides, in the embodiment shown in FIG. 4, at an upstream side of the development transfer unit 126 located upstream of the fixing unit 127, the sheet is aligned by the register rollers 207 a and 207 b before the development and transfer are performed on the sheet. The paper thickness detection is performed at a further upstream side of the register rollers 207 a and 207 b. Accordingly, there is a sufficient time from the detection of the paper thickness in the paper thickness detection unit 124 to the change of the fixing temperature, and excellent fixing can always be performed according to the paper thickness. Besides, since the sheet is aligned by the register roller before the development and transfer are performed, after the paper thickness is detected, the condition of the development and transfer, such as voltage, can be easily set in accordance with the paper thickness, and an excellent image can always be obtained.

In the embodiment, the description is given to the case where the invention is applied to the multi-function color copier. However, the invention can be applied not only to the multi-function color copier, but also to another image forming apparatus, such as a normal copier, a printer or a facsimile, which includes an image generating unit to generate an image to be printed on a recording sheet and specifies the type of a sheet to be printed.

Obviously, many modifications and variations of this invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, this invention may be practiced otherwise than as specification. 

1. An image forming apparatus with a paper thickness detection unit, comprising: a paper feed cassette to contain a sheet on which an image is to be printed; a pair of conveyance rollers to nip and convey the sheet taken out from the paper feed cassette; a paper thickness detection unit that is provided downstream of the sheet conveyed by the pair of conveyance rollers and near a nip point between the pair of conveyance rollers, has a rotator brought into contact with the sheet, and detects a thickness of the sheet by a shift generated when the sheet contacts with the rotator; a register roller to align the sheet after the thickness of the sheet is detected by the paper thickness detection unit; and a print ejection mechanism that prints and fixes an image to the sheet passing through the register roller and then ejects it.
 2. The apparatus according to claim 1, wherein the paper thickness detection unit detects a shift amount of the rotator as a voltage value and detects the paper thickness.
 3. The apparatus according to claim 2, wherein the paper thickness detection unit samples the voltage value to obtain an average value.
 4. The apparatus according to claim 3, wherein the paper thickness detection unit detects the paper thickness as a difference between a voltage when the sheet does not contact with the rotator and a voltage when the sheet contacts with the rotator.
 5. The apparatus according to claim 4, wherein a distance D from a nip point between the pair of conveyance rollers to a point where the rotator contacts with the sheet is within a range of about 0 mm to 10 mm.
 6. The apparatus according to claim 5, wherein a pressing load P at which the rotator contacts with the sheet is within a range of about 60 g to 140 g.
 7. The apparatus according to claim 4, wherein a fixing temperature of the image in the print ejection mechanism is changed according to the paper thickness detected in the paper thickness detection unit.
 8. An image forming apparatus with a paper thickness detection unit, comprising: a plurality of paper feed cassettes to contain sheets on which images are to be printed; a conveyance drive roller and a conveyance driven roller that nip and convey the sheet taken out from one of the plurality of paper feed cassettes; a first conveyance guide provided at a side of the conveyance drive roller and a second conveyance guide provided at a side of the conveyance driven roller, which have openings at portions where the conveyance drive roller and the conveyance driven roller contact with each other, and guide the sheet conveyed by these rollers, a paper thickness detection unit that is provided downstream of the sheet conveyed by the conveyance drive roller and the conveyance driven roller and near a nip point between the conveyance drive roller and the conveyance driven roller, includes a rotator brought into contact with the first conveyance guide and the sheet at a specified pressing load, and detects a thickness of the sheet by a shift generated when the sheet contacts with the rotator; a register roller to align the sheet after the thickness of the sheet is detected by the paper thickness detection unit; and a print ejection mechanism that prints and fixes an image to the sheet passing through the register roller and then ejects it.
 9. The apparatus according to claim 8, wherein the paper thickness detection unit detects a shift amount of the rotator as a voltage value and detects the paper thickness.
 10. The apparatus according to claim 9, wherein the paper thickness detection unit samples the voltage value to obtain an average value.
 11. The apparatus according to claim 10, wherein the paper thickness detection unit detects the paper thickness as a difference between a voltage when the sheet does not contact with the rotator and a voltage when the sheet contacts with the rotator.
 12. The apparatus according to claim 11, wherein a distance D from a nip point between the pair of conveyance rollers to a point where the rotator contacts with the sheet is within a range of about 0 mm to 10 mm.
 13. The apparatus according to claim 12, wherein a pressing load P at which the rotator contacts with the sheet is within a range of about 60 g to 140 g.
 14. The apparatus according to claim 11, wherein a fixing temperature of the image in the print ejection mechanism is changed according to the paper thickness detected in the paper thickness detection unit.
 15. An image forming apparatus with a paper thickness detection unit, comprising: a plurality of paper feed cassettes to contain sheets on which images are to be printed; a conveyance drive roller and a conveyance driven roller that nip and convey the sheet taken out from one of the plurality of paper feed cassettes; a first conveyance guide provided at a side of the conveyance drive roller and a second conveyance guide provided at a side of the conveyance driven roller, which have openings at portions where the conveyance drive roller and the conveyance driven roller contact with each other, and guide the sheet conveyed by these rollers, a paper thickness detection unit that is provided downstream of the sheet conveyed by the conveyance drive roller and the conveyance driven roller and near a nip point between the conveyance drive roller and the conveyance driven roller, includes a rotator brought into contact with the first conveyance guide and the sheet at a specified pressing load through an opening of the second conveyance guide, and detects a thickness of the sheet by a shift generated when the sheet contacts with the rotator; a register roller to align the sheet after the thickness of the sheet is detected by the paper thickness detection unit; and a print ejection mechanism that prints and fixes an image to the sheet passing through the register roller and then ejects it.
 16. The apparatus according to claim 15, wherein the paper thickness detection unit detects a shift amount of the rotator as a voltage value, samples the voltage value to obtain an average value, and detects the paper thickness.
 17. The apparatus according to claim 16, wherein the paper thickness detection unit detects the paper thickness as a difference between a voltage when the sheet does not contact with the rotator and a voltage when the sheet contacts with the rotator.
 18. The apparatus according to claim 17, wherein a fixing temperature of the image in the print ejection mechanism is changed according to the paper thickness detected in the paper thickness detection unit.
 19. An image forming method, comprising: taking out a sheet on which an image is to be printed from a paper feed cassette; nipping and conveying the taken-out sheet by a pair of conveyance rollers; detecting a thickness of the sheet by a shift generated when a rotator, which is provided downstream of the pair of conveyance rollers to nip and convey the sheet and near a nip point between the pair of conveyance rollers and is brought into contact with the sheet, contacts with the sheet; aligning the sheet by a register roller after the thickness of the sheet is detected; and ejecting the sheet after an image is printed and fixed to the sheet passing through the register roller.
 20. The method according to claim 19, wherein at the detecting of the thickness of the sheet, a shift amount of the rotator is detected as a voltage value, the voltage value is sampled to obtain an average value, and the paper thickness is detected. 